Alpha: Restore frame pointer last in `builtin_longjmp' [PR64242]

Add similar arrangements to `builtin_longjmp' for Alpha as with commit
71b14428 ("re PR middle-end/64242 (Longjmp expansion incorrect)")
and commit 511ed59d ("Fix PR64242 - Longjmp expansion incorrect"),
so as to restore the frame pointer last, so that accesses to a local
buffer supplied can still be fulfilled with memory accesses via the
original frame pointer, fixing:

FAIL: gcc.c-torture/execute/pr64242.c   -O0  execution test
FAIL: gcc.c-torture/execute/pr64242.c   -O1  execution test
FAIL: gcc.c-torture/execute/pr64242.c   -O2  execution test
FAIL: gcc.c-torture/execute/pr64242.c   -O3 -g  execution test
FAIL: gcc.c-torture/execute/pr64242.c   -Os  execution test
FAIL: gcc.c-torture/execute/pr64242.c   -O2 -flto -fno-use-linker-plugin -flto-partition=none  execution test
FAIL: gcc.c-torture/execute/pr64242.c   -O2 -flto -fuse-linker-plugin -fno-fat-lto-objects  execution test

and adding no regressions in `alpha-linux-gnu' testing.

	gcc/
	PR middle-end/64242
	* config/alpha/alpha.md (`builtin_longjmp'): Restore frame
	pointer last.  Add frame clobber and schedule blockage.