Merge #17569: build: Allow export of environ symbols and work around rv64 toolchain issue

eafd259367 build: Add NX workaround for RV64 (Wladimir J. van der Laan)
f6e42256fe build: Allow export of environ symbols (Wladimir J. van der Laan)

Pull request description:

  This export was introduced in #17270 which added
  ```
  //! Necessary on some platforms
  extern char** environ;
  ```
  This should (finally) make the gitian build pass again (fix issue #17525.).

  Built on top of #17538 which should be merged first.

Top commit has no ACKs.

Tree-SHA512: 5c2054d52d0957aec3dc945b76d8e219187d22dc03889e7a88fb76049bf8e4a3e9f4da00dd1e9dd0351211f8e70d1a1b8ad7244f0348dab698e9d14b9d0c0bd4

contrib/devtools/symbol-check.py

@@ -47,7 +47,8 @@ MAX_VERSIONS = {
 
 # Ignore symbols that are exported as part of every executable
 IGNORE_EXPORTS = {
-'_edata', '_end', '__end__', '_init', '__bss_start', '__bss_start__', '_bss_end__', '__bss_end__', '_fini', '_IO_stdin_used', 'stdin', 'stdout', 'stderr'
+'_edata', '_end', '__end__', '_init', '__bss_start', '__bss_start__', '_bss_end__', '__bss_end__', '_fini', '_IO_stdin_used', 'stdin', 'stdout', 'stderr',
+'environ', '_environ', '__environ',
 }
 READELF_CMD = os.getenv('READELF', '/usr/bin/readelf')
 CPPFILT_CMD = os.getenv('CPPFILT', '/usr/bin/c++filt')

contrib/gitian-descriptors/gitian-linux.yml

@@ -46,7 +46,7 @@ script: |
   FAKETIME_PROGS="date ar ranlib nm"
   HOST_CFLAGS="-O2 -g"
   HOST_CXXFLAGS="-O2 -g"
-  HOST_LDFLAGS=-static-libstdc++
+  HOST_LDFLAGS_BASE="-static-libstdc++"
 
   export QT_RCC_TEST=1
   export QT_RCC_SOURCE_DATE_OVERRIDE=1
@@ -160,6 +160,13 @@ script: |
   # Extract the release tarball into a dir for each host and build
   for i in ${HOSTS}; do
     export PATH=${BASEPREFIX}/${i}/native/bin:${ORIGPATH}
+    if [ "${i}" = "riscv64-linux-gnu" ]; then
+      # Workaround for https://bugs.launchpad.net/ubuntu/+source/gcc-8-cross-ports/+bug/1853740
+      # TODO: remove this when no longer needed
+      HOST_LDFLAGS="${HOST_LDFLAGS_BASE} -Wl,-z,noexecstack"
+    else
+      HOST_LDFLAGS="${HOST_LDFLAGS_BASE}"
+    fi
     mkdir -p distsrc-${i}
     cd distsrc-${i}
     INSTALLPATH=`pwd`/installed/${DISTNAME}