rust191: update to rust version 1.91.1.

Pkgsrc changes:
 * Re-enable the powerpc build in do-cross.mk.
 * Update version & checksums.

Noteable failures at the time of commit:
 * The cross-build for armv6 fails, reported upstream at
   rust-lang/rust#148464

Upstream changes relative to 1.91.0:

Version 1.91.1 (2025-11-10)
===========================

- [Enable file locking support in illumos]
  (rust-lang/rust#148322).
  This fixes Cargo not locking the build directory on illumos.

- [Fix `wasm_import_module` attribute cross-crate]
  (rust-lang/rust#148363).
  This fixes linker errors on WASM targets.

Author: he32

rust191/do-cross.mk

@@ -8,7 +8,7 @@ V_NOREV!=	make show-var VARNAME=PKGVERSION_NOREV
 SHORT_TARGETS+=		armv7
 #SHORT_TARGETS+=		armv6
 SHORT_TARGETS+=		sparc64
-#SHORT_TARGETS+=		powerpc
+SHORT_TARGETS+=		powerpc
 SHORT_TARGETS+=		arm64
 SHORT_TARGETS+=		arm64_be
 SHORT_TARGETS+=		i386