gdb: use std::vector<> to hold on blocks in struct blockvector

This patch changes blockvector to be allocated on the heap (using 'new')
and changes internal implementation to use std::vector<> rather than
flexible array to add blocks to existing blockvector. This is needed for
lazy CU expansion and for Python API to build objfiles, compunits and
symtabs dynamically (similarly to JIT reader API).

The downside is higher memory consumption. The size of std::vector is
24 bytes (GCC 14) compared to 8 bytes used currently to store the number
of blocks (m_num_blocks). Stopping gdb at its main(), followed by
"maint expand-symtabs" results in 4593 compunit symtabs so in this case
the overhead is 16*4593 = 73488 bytes which I hope is acceptable.

While at it, add blockvector::append_block() to add more block at the
end of block vector. This is currently used only in mdebugread.c.

Approved-By: Tom Tromey <tom@tromey.com>