Some of the masks are wrong, and the naming is confusing.
There are two basic cases we really care about:
1. Stacking a new rounding mode when running certain
sections of code, and pausing exception handling.
2. Likewise, but discarding any exceptions which occur
while running under the new rounding mode.
libc_feholdexcept_setround_ppc_ctx has been removed as it basically
does the same thing as libc_feholdsetround_ppc_ctx but also clearing
any sticky bits. The restore behavior is what differentiates these
two cases as the SET_RESTORE_ROUND{,_NOEX} macros will either merge
or discard all exceptions occurring during scope of their usage.
Likewise, there are a number of routines to swap, replace,
or merge FP environments. This change reduces much of
the common and sometimes wrong code.
Tested on ppc64le, with results before and after.
84d74e427a