When dynamic linking, R15 may be clobbered by a global variable access

[ale-linux]

The problem can be reproduced by running

go build -linkshared ./...

Output:

asm: fp_arithmetic_x86.s:1427: when dynamic linking, R15 is clobbered by a global variable access and is used here: 00895 (src/github.com/kilic/bls12-381/fp_arithmetic_x86.s:1427)  CMOVQCC R12, R15
asm: fp_arithmetic_x86.s:2142: when dynamic linking, R15 is clobbered by a global variable access and is used here: 01198 (src/github.com/kilic/bls12-381/fp_arithmetic_x86.s:2142)  CMOVQCC R8, R15