Disable hexagon-unknown-linux-musl testing for now

`compiler-builtins` seems to be providing duplicate symbols on Hexagon.
This may be because some symbols have always been defined in assembly,
but are now also provided by the weak math symbols.

The fix for this is probably to not provide any math symbols on Hexagon
that would duplicate the assembly version. However, to avoid going
through another nightly release cycle, disable it temporarially instead.

Link: rust-lang#3797 (comment)
(backport <rust-lang#3797>)
(cherry picked from commit 77e9d06)

Author: tgross35

Cargo.toml

@@ -46,7 +46,8 @@ targets = [
     "armv7-wrs-vxworks-eabihf",
     "armv7r-none-eabi",
     "armv7r-none-eabihf",
-    "hexagon-unknown-linux-musl",
+    # FIXME(hexagon): excluded due to duplicate symbol errors
+    # "hexagon-unknown-linux-musl",
     "i586-pc-windows-msvc",
     "i586-unknown-linux-gnu",
     "i586-unknown-linux-musl",

ci/build.sh

@@ -212,6 +212,8 @@ for TARGET in $TARGETS; do
 done
 
 # Targets which are not available via rustup and must be built with -Zbuild-std
+# FIXME(hexagon): hexagon-unknown-linux-musl should be tested but currently has
+# duplicate symbol errors from `compiler_builtins`.
 RUST_LINUX_NO_CORE_TARGETS="\
 aarch64-pc-windows-msvc \
 aarch64-unknown-freebsd \
@@ -224,7 +226,6 @@ armebv7r-none-eabihf \
 armv7-wrs-vxworks-eabihf \
 armv7r-none-eabi \
 armv7r-none-eabihf \
-hexagon-unknown-linux-musl \
 i586-pc-windows-msvc \
 i686-pc-windows-msvc \
 i686-unknown-haiku \