Relax GOT indirection into PC-relative addressing

Some psABIs define the linker optimizations to relax a GOT load into a
PC-relative address materialization. AArch64 [1] allows the linker to
rewrite ADRP+ADD into ADR. x86-64 does the same thing with the
`R_X86_64_GOTPCRELX` and `R_X86_64_REX_GOTPCRELX` relocations.

In our case, we have lots of AUIPC+LD instruction pairs to load an
address from the GOT in our RISC-V programs because `la` assembly pseudo
instruction is expanded to that instruction pair. If the PC-relative
address loaded by the instruction pair is a link-time constant, we can
rewrite the instructions with AUIPC+ADDI to directly materialize the
value into a register, which eliminates one memory load.

[1] https://github.com/ARM-software/abi-aa/blob/844a79fd4c77252a11342709e3b27b2c9f590cf1/aaelf64/aaelf64.rst#relocation-optimization

riscv-elf.adoc

@@ -1567,6 +1567,79 @@ optimize code size and performance of the symbol accessing.
 
 NOTE: Tag_RISCV_x3_reg_usage is treated as 0 if it is not present.
 
+==== GOT load relaxation
+
+  Target Relocation:: R_RISCV_GOT_HI20, R_RISCV_PCREL_LO12_I
+
+  Description:: This relaxation can relax a GOT indirection into load
+  immediate or PC-relative addressing. This relaxation is intended to
+  optimize the `la`/`lga` assembly pseudo-instruction, which loads a
+  symbol's address from a GOT entry with an `auipc` + `ld` instruction
+  pair.
+
+  Condition::
+  - Both `R_RISCV_GOT_HI20` and `R_RISCV_PCREL_LO12_I` are marked
+  with `R_RISCV_RELAX`.
+
+  - `R_RISCV_GOT_HI20` refers to the location 4 bytes before where
+  `R_RISCV_PCREL_LO12_I` points.
+
+  - The symbol pointed to by `R_RISCV_PCREL_LO12_I` is at the location to
+  which `R_RISCV_GOT_HI20` refers.
+
+  - The symbol's address is a link-time constant.
+
+  - If the symbol is absolute, its address is within `0x0` ~ `0x7ff`.
+  Note that an unresolved weak symbol satisfies this condition because
+  such symbol is handled as if it were an absolute symbol at address 0.
+
+  - If the symbol is relative, the offset between the location to
+  which `R_RISCV_GOT_HI20` refers and the target symbol is within +-2GiB.
+
+  Relaxation::
+  - Instruction sequence associated with `R_RISCV_GOT_HI20` and
+  `R_RISCV_PCREL_LO12_I` can be rewritten to an `c.li`, `addi` or
+  `auipc` + `addi` to materialize the symbol's address directly in a
+  register, eliminating a load from the GOT.
+
+  - If this relaxation eliminates all references to the symbol's GOT slot,
+  the linker may opt not to create a GOT slot for that symbol.
+
+  Example::
++
+--
+Relaxation candidate:
+[,asm]
+----
+label:
+    auipc   t0, 0      # R_RISCV_GOT_HI20 (symbol), R_RISCV_RELAX
+    ld      t0, 0(t0)  # R_RISCV_PCREL_LO12_I (label), R_RISCV_RELAX
+----
+
+Relaxation result (absolute symbol whose address is within `0x0` ~ `0x1f`
+and if the RVC instruction is permitted):
+
+[,asm]
+----
+    c.lui   t0, <symbol-value>
+----
+
+Relaxation result (absolute symbol whose address is larger than `0x1f`
+or if the RVC instruction is not permitted):
+
+[,asm]
+----
+    addi    t0, zero, <symbol-value>
+----
+
+Relaxation result (relative symbol):
+[,asm]
+----
+    auipc   t0, <hi>
+    addi    t0, t0, <lo>
+----
+--
+
 ==== Zero-page Relaxation
 
   Target Relocation:: R_RISCV_HI20, R_RISCV_LO12_I, R_RISCV_LO12_S