Increase maximum registers allowed in an asm block

[MichaelBell]

By default, gcc only allows 30 registers to be used in an inline asm block. Especially when using hardware loops it's nice to be able to replace fairly large blocks of code with asm, and I find the 30 register limit a pain given the epiphany has plenty of registers. This is made worse because read/write registers count twice against this limit!

I've done limited testing, but this change seems to work for me:
MichaelBell/epiphany-gcc@0dc1a94

I can submit a pull request if you want, but would like someone with more gcc experience to do a bit of verification first!