[build2] Executable ELF files not installed with executable file mode

[Matthew Krupcale]

Hello,

The install operation appears to use the config.install.mode mode on
every target file installed, but I think it would be appropriate to
install executables such as ELF files (shared libraries and programs)
and scripts with executable file mode. This is most simply achieved
(at least on Linux) by not passing the "-m mode" argument to
install(1) since it will default to rwxr-xr-x. This doesn't respect
the config.install.mode rw permission bits, though, so perhaps you
could use a bitwise OR to get the desired permissions which are still
executable.

This showed up as an error in the final stages of the RPM for
build2[1] because it was not able to find the debug info for the
installed files since the ELF files did not have executable
permissions (the find-debuginfo.sh script[2] filters files which have
executable permissions).

Best,
Matthew

[1] https://copr-be.cloud.fedoraproject.org/results/mkrupcale/build2/fedora-28-x86_64/00798329-build2/builder-live.log
[2] https://github.com/rpm-software-management/rpm/blob/master/scripts/find-debuginfo.sh