Revert "_build.sh: drop workaround for clang-rt on riscv64 linking issue"

This reverts commit 13c7de8.

This isn't resolved in trixie or testing:
https://github.com/curl/curl-for-win/actions/runs/17181424023/job/48744299228#step:3:6459

Seem in these two jobs:
linux-glibc-debian-testing-llvm (this is pretty much the same as trixie at the time of this commit
linux-glibc-debian-trixie-llvm

Author: vszakats

_build.sh

@@ -1094,7 +1094,16 @@ build_single_target() {
     # it requires manually installing package `libclang-rt-17-dev`.
     elif [ "${_OS}" = 'linux' ] && [ "${_DISTRO}" = 'debian' ]; then
       if [ "${unamem}" = "${_machine}" ] || [ -d 'my-pkg/usr/lib/clang' ]; then
-        _CCRT='clang-rt'
+        # FIXME: This combination fails to link with clang-rt, due to:
+        #        ld.lld-17: error: relocation R_RISCV_PCREL_HI20 cannot be used against symbol '__global_pointer$'; recompile with -fPIC
+        #        >>> defined in /usr/riscv64-linux-gnu/lib/Scrt1.o
+        #        >>> referenced by /usr/riscv64-linux-gnu/lib/Scrt1.o:(.text+0x2C)
+        #        Our workaround is to fall back to gcc parts for this.
+        if [ "${_CRT}" = 'gnu' ] && [ "${_CPU}" = 'r64' ]; then
+          :
+        else
+          _CCRT='clang-rt'
+        fi
       fi
     fi
   fi