Update installation scripts

[sbalint98]

This PR contains the following changes to the installation scripts:

• The default LLVM version is set to 11.0.1 Use HIPSYCL_LLVM_DIR to find the LLVM executable and LLVM include paths.
• The naming problem reported in issue install-llvm.sh doesn't install in the right place #676 has been corrected, now the user needs to set HIPSYCL_INSTALL_PREFIX to control the installation directory instead of INSTALL_PREFIX.
• In the install-llvm.sh script instead of checking out branches from the LLVM repo tags are used. This allows controlling more precisely the LLVM version.
• By default the install-hipsycl.sh script installs from the develop branch of the hipSYCL repo
• During hipSYCL compilation all available cores are used ( make -j nproc` )
• The LLVM_ENABLE_DUMP and LLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN configuration options have been disabled when configuring LLVM

[sbalint98]

@jeffhammond Can you have a look if these changes solve the issue?