x86: efi: app: Generate Microsoft PE format compliant image

Per Microsoft PE Format documentation [1], PointerToSymbolTable and
NumberOfSymbols should be zero for an image in the COFF file header.
Currently U-Boot is generating u-boot-app.efi in which these two
members are not zero.

This updates the build rules to tell linker to remove the symbol
table completely so that we can generate compliant *.efi images.

[1] https://docs.microsoft.com/zh-cn/windows/desktop/Debug/pe-format



Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>