[Sanitizer][RISCV] Fix redefinition of REG_SP

The include header sys/ucontext.h already defines REG_SP as 2, causing
redefinition warnings during compilation. This patch fixes that issue.
(We also can't just use the numerical definition provided by the header,
as REG_SP is used in this file this refers to a struct field.)

Differential Revision: https://reviews.llvm.org/D90934