Use a single 'ld' to read the jmpbuf magic values instead of 'la; ld'.

[jhb]

Summary

This saves an instruction in each case.

Test Plan

• tests in /usr/tests/lib/libc/setjmp pass

[jrtc27]

This is particularly beneficial as, for PIC code, which this will almost always be, la expands to auipc/ld to index the GOT, so we save a level of unnecessary indirection too. (This could have been lla instead to save the indirection but still take 2 instructions just to do the address calculation.)