patch 9.0.1835: Perl interface has problems with load PL_current_context

patch 9.0.1835: Perl interface has problems with load PL_current_context

Problem:  Perl interface has problems with load PL_current_context
Solution: Fix Perl interface to load PL_current_context from library

In #12914, in order to fix an issue with Perl 5.36 dynamic builds, (that
version introduced a thread-local `PL_current_context`), the file added
the variable manually so we can satisfy the linker. However, the
variable is a different one from the one in the library, so there could
be unpredictable behavior. Instead, just use `dlsym` to load the context
from the library. The fact that it's thread-local doesn't matter too
much to us because Vim's interface is single-threaded so it will work
properly.

closes: #12996

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Yee Cheng Chin <ychin.git@gmail.com>