lang/ocaml: fix build on powerpc

ld: error: relocation R_PPC_ADDR16_HA cannot be used against symbol 'caml_program'; recompile with -fPIC
>>> defined in power_libasmrunpic.o
>>> referenced by power_libasmrunpic.o:(caml_start_program)

ld: error: relocation R_PPC_ADDR16_LO cannot be used against symbol 'caml_program'; recompile with -fPIC
>>> defined in power_libasmrunpic.o
>>> referenced by power_libasmrunpic.o:(caml_start_program)

ld: error: relocation R_PPC_ADDR16_HA cannot be used against symbol 'caml_apply2'; recompile with -fPIC
>>> defined in power_libasmrunpic.o
>>> referenced by power_libasmrunpic.o:(caml_callback2_asm)

ld: error: relocation R_PPC_ADDR16_LO cannot be used against symbol 'caml_apply2'; recompile with -fPIC
>>> defined in power_libasmrunpic.o
>>> referenced by power_libasmrunpic.o:(caml_callback2_asm)

ld: error: relocation R_PPC_ADDR16_HA cannot be used against symbol 'caml_apply3'; recompile with -fPIC
>>> defined in power_libasmrunpic.o
>>> referenced by power_libasmrunpic.o:(caml_callback3_asm)

ld: error: relocation R_PPC_ADDR16_LO cannot be used against symbol 'caml_apply3'; recompile with -fPIC
>>> defined in power_libasmrunpic.o
>>> referenced by power_libasmrunpic.o:(caml_callback3_asm)
cc: error: linker command failed with exit code 1 (use -v to see invocation)