Rollup merge of rust-lang#32342 - petevine:patch-1, r=alexcrichton

Use explicit -march flags in the i586 mk file

`-march` should definitely go last, after the environment `$C(XX)FLAGS`, or it's going to remain brittle.

This should fix cross-compilation issues on x86_64 (possibly x86 too on some newer distros) - it was far too assuming of me to expect you really had to want i586 using your own flags.

mk/cfg/i586-unknown-linux-gnu.mk

@@ -7,9 +7,9 @@ CFG_LIB_NAME_i586-unknown-linux-gnu=lib$(1).so
 CFG_STATIC_LIB_NAME_i586-unknown-linux-gnu=lib$(1).a
 CFG_LIB_GLOB_i586-unknown-linux-gnu=lib$(1)-*.so
 CFG_LIB_DSYM_GLOB_i586-unknown-linux-gnu=lib$(1)-*.dylib.dSYM
-CFG_JEMALLOC_CFLAGS_i586-unknown-linux-gnu := -m32 $(CFLAGS)
-CFG_GCCISH_CFLAGS_i586-unknown-linux-gnu := -Wall -Werror -g -fPIC -m32 $(CFLAGS)
-CFG_GCCISH_CXXFLAGS_i586-unknown-linux-gnu := -fno-rtti $(CXXFLAGS)
+CFG_JEMALLOC_CFLAGS_i586-unknown-linux-gnu := -m32 $(CFLAGS) -march=pentium
+CFG_GCCISH_CFLAGS_i586-unknown-linux-gnu := -Wall -Werror -g -fPIC -m32 $(CFLAGS) -march=pentium
+CFG_GCCISH_CXXFLAGS_i586-unknown-linux-gnu := -fno-rtti $(CXXFLAGS) -march=pentium
 CFG_GCCISH_LINK_FLAGS_i586-unknown-linux-gnu := -shared -fPIC -ldl -pthread  -lrt -g -m32
 CFG_GCCISH_DEF_FLAG_i586-unknown-linux-gnu := -Wl,--export-dynamic,--dynamic-list=
 CFG_LLC_FLAGS_i586-unknown-linux-gnu :=