gcc-4.9: port AmigaOS specific code from gcc 2.95.3

[cahirwpz]

gcc-amigaos
libnix

Code generation options:

• -msmall-code - all branches use only 16-bit displacement, thus all code must fit into 64KiB,
• -fbaserel - data and bss segments are addressed through a4 register with 16-bit displacement, thus merged size of data and bss must be below 64KiB,
• -fbaserel32- data and bss segments are addressed through a4 register with displacement of 32-bit.

Function attributes:

• saveds - restore a4 register in any function called from outside where the register could've been trashed,
• regparm - pass arguments to function in registers if possible,
• interrupt - use rte instead of rts and save scratch registers on stack,
• amiga_interrupt - set condition codes before returning to AmigaOS interrupt handler.

Explicit register specification:

• modify parser to support asm keyword after function argument,
• modify code that generates function prologue / epilogue.