set rust crate lto to thin to not use fatlto for gkrust which fails sometimes

additionally, disable lto on riscv64 altogether

--- a/config/makefiles/rust.mk
+++ b/config/makefiles/rust.mk
@@ -91,11 +91,13 @@
 ifndef rustflags_sancov
 # Never enable when coverage is enabled to work around https://github.com/rust-lang/rust/issues/90045.
 ifndef MOZ_CODE_COVERAGE
+ifeq (,$(findstring riscv64,$(RUST_TARGET)))
 ifeq (,$(findstring gkrust_gtest,$(RUST_LIBRARY_FILE)))
-cargo_rustc_flags += -Clto$(if $(filter full,$(MOZ_LTO_RUST_CROSS)),=fat)
+cargo_rustc_flags += -Clto=thin
 endif
 # We need -Cembed-bitcode=yes for all crates when using -Clto.
 RUSTFLAGS += -Cembed-bitcode=yes
+endif
 endif
 endif
 endif