The following are ABI register usage notes accumulated with development and porting of musl to various archs:

aarch64

• x30 lr
• x31 zr or pc

arm

• r0-r3 args/ret
• r4-r11 saved
• r12 temp (ip scratch)
• r13 sp
• r14 lr
• r15 pc

microblaze

• r0 zero
• r1 sp
• r2 ro sda
• r3-r4 ret
• r5-r10 args
• r11-12 temp
• r13 rw sda
• r14 syscall ret addr
• r15 function ret addr
• r16 break ret addr
• r17 exception ret addr
• r18 assembler temp
• r19-r20 saved
• r21 thread pointer
• r21-r31 saved

mips

• $0 zero
• $1 at (assembler temp)
• $2-$3 ret (aka v0-v1)
• $4-$7 args (aka a0-a3)
• $8-$15 temp (aka t0-t7)
• $16-$23 saved (aka s0-s7)
• $24 temp (aka t8)
• $25 function call addr (aka t9)
• $26-$27 kernel use
• $28 gp, call-clobbered
• $29 sp
• $30 fp
• $31 ra

Source: http://www.inf.ed.ac.uk/teaching/courses/car/Notes/slide03.pdf

or1k (OpenRISC)

• r0 zero
• r1 sp
• r2 fp
• r3-r8 args
• r9 lr
• r11,r12 retval (lo,hi)
• r10 thread pointer
• r14-r30(even) saved
• r13-r31(odd) temp

Source: openrisc-arch-1.1-rev0.pdf, p.335

powerpc

• 0 temp
• 1 sp
• 2 toc (thread pointer)
• 3 ret, arg0
• 4-10 args
• 11-12 temp
• 13-31 saved
• lr (not gpr; use mtlr/mflr)
• 30 got
• 11 plt temp
• ctr plt temp

Sources: http://www.csd.uwo.ca/~mburrel/stuff/ppc-asm.html, http://devpit.org/wiki/Debugging_PowerPC_ELF_Binaries

sh (SuperH)

• r0-r3 ret
• r2 struct ret addr
• r4-r7 args
• r8-r13 saved
• r12 gp (saved)
• r14 fp (saved)
• r15 sp
• pr lr
• gbr thread ptr

Source: http://www.st.com/st-web-ui/static/active/en/resource/technical/document/reference_manual/CD17839242.pdf, p.9