package/musl: move riscv64 register index constant definitions

package/musl: move riscv64 register index constant definitions

The riscv64 build of libsigsegv using musl fails due to a missing
definition for REG_SP. This constant is used to index the __gregs
array in the ucontext_t structure.

This fix moves the musl defintion of REG_SP (and others) from
arch/riscv64/bits/reg.h to arch/riscv64/bits/signal.h
so that it is picked up correctly.

The patch was downloaded from upstream:
https://git.musl-libc.org/cgit/musl/commit/?id=329e79299daaa994b8e75941331a1093051ea5d9

Fixes:
http://autobuild.buildroot.org/results/8fcd5cb912e513ac08a81e3ee726e29ac22212bb/

Signed-off-by: Mark Corbin <mark@dibsco.co.uk>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>